A conventional way of forming an integrated optics waveguide is to diffuse titanium into the surface of a lithium niobate crystal, such that the titanium indiffused region forms a linear channel in the crystal parallel to the crystal surface. To create a single mode optical waveguide, typical titanium channel widths are 2-4 microns. The titanium increases the index of refraction of the lithium niobate by about 0.01 (from 2.10 to 2.11), and this index of refraction shift is sufficient to confine an optical signal to the waveguide. The waveguide is typically fabricated by depositing a strip of titanium on the lithium niobate crystal using photolithography techniques, and then diffusing the titanium into the crystal at a temperature of about 1,000.degree. C.
There are a number of applications in which it would be desirable to produce large and controllable variations in the effective index of refraction of an integrated optics waveguide. One class of such applications involves the use of interferometers having unequal arm lengths. By way of example, FIG. 1 schematically illustrates a Mach-Zehnder interferometer of a type commonly used in integrated optics applications. The interferometer includes input waveguide 12, arms (i.e., waveguides) 14 and 16, and output waveguide 18. Arms 14 and 16 are coupled to the input and output waveguides by Y-couplers 20 and 22 respectively. Y-coupler 20 divides an optical input signal on input waveguides 12 between arms 14 and 16, and Y-coupler 22 combines the optical signals on arms 14 and 16 onto output waveguide 18.
If the optical path lengths of arms 14 and 16 differ from one another, then the optical signals from the arms will be phase shifted (or equivalently, time delayed) with respect to one another when they are brought together at Y-coupler 22. For the case in which the arms have different physical path lengths, the amount of the phase shift will be equal to 2.pi.n.DELTA.d/.lambda., where n is the effective index of refraction of the waveguides, .DELTA.d is the path length difference, and .lambda. is the wavelength of the light. An example of an application of unequal arm length interferometers is coherence multiplexing. In a coherence multiplexing system, each information bearing channel is associated with an interferometer having a particular optical path length difference, and the optical path length differences must differ from one another by an amount greater than the coherence length of the light. Thus the ability to create a wide range of optical path length differences increases the number of channels that may be multiplexed.
FIG. 2 illustrates one geometric means of obtaining a physical path length differences in a Mach-Zehnder interferometer. The illustrated interferometer includes input waveguide 30, arms 32 and 34, and output waveguide 36. To avoid radiating energy out of the waveguide, the angle 38 between arms 32 and 34 is limited to about 1,2.degree.. With this limitation, for an interferometer having a length L of 20 mm, the maximum resulting physical path length difference is 6.7 microns. FIG. 3 illustrates an interferometer in which arm 40 has the shape of a curve that takes the longest possible route between two points a distance L apart. For this arrangement, to avoid unacceptable radiation loss, the angle 44 between arms 40 and 42 is limited to 1.2.degree., and arm 40 has a minimum radius of curvature of 110 mm. The result is that for a length L of 20 mm, the maximum physical path length differences is 25.2 microns.
An optical path length difference between two interferometer arms can be produced not only by fabricating the arms to have different physical lengths, but also by causing the arms to have different effective indices of refraction. The usual method of varying the index of refraction of a waveguide is to fabricate the waveguide in an electro-optic material such as lithium niobate, deposit electrodes adjacent a portion of the waveguide, and then apply a voltage to the electrodes. The electric field produced by the applied voltage rotates the index of refraction ellipsoid slightly, thereby changing the effective index of refraction along the direction that light passes through the waveguide. This in turn changes the waveguide optical path length by .DELTA.and, .DELTA.n being the index of refraction change, and d being the length of the waveguide section over which the field is applied.
The phase shift that can be produced via the electro-optic approach is 2.pi..DELTA.nd/.lambda., and this expression may be evaluated from known crystal properties or from the properties of available commercial integrated optics devices. For example, a Mach-Zehnder interferometer sold by Crystal Technology produces a phase shift of 2.pi. for light having a wavelength of 850 nanometers, from electrodes 4 mm long. Over a 20 mm length, the optical path difference is 4.3 microns at a maximum. Using the practical 1 MV/m maximum electric field in the theoretical 164 pm/V electro-optic coefficient, such as interferometer of 14 mm electrode length in 20 mm overall length would produce only 5.3 microns of optical path difference by application of an electric field.